Changing 6ohm cab to 4ohm

[la bam]

Hi all,

Just wondering how difficult it is (for a professional) to change a 6ohm cab into a 4ohm cab?

It's it a complex job, or just changing a few components? Or is it more has than it's worth?

I'm thinking going valve amp and it needs to see 4ohm.

[bassace]

6 ohm?

[obbm]

What is the cab?

[la bam]

It's a markbass cl106. Sealed cab.

I love it, but it may as well be 4ohm as I'm never going to use any extra cab with it. And at 6ohm I couldn't use with a valve amp that only does 4 or 2 ohm.

[James Nada]

A quick Google tells me that's a 6x10 cab.  On the assumption that as most production bass drivers are 4 or 8 ohms,  each driver is wired to get equal power, and as your cab is 6ohms, I'd guess you have 4 ohm drivers.  (Thats each "pair" of drivers wired in parallel to give 2 ohms, and the three "pairs" are in series to give 6 ohms).

So, that means you could wire the speakers to give 2.66 ohms, which is in the ballpark you need.  (That's wiring "pairs" of drivers in series to give 8 ohms, and wiring all those in parallel to give 2.66 ohms).

You'd need to be sure of each drivers impedance though, best trying to look at the back of one.  Probably best if someone checksy maths and assumptions though - I've had a couple of beers!

Edited December 27, 2018 by James Nada

[obbm]

Whichever way you do the maths with any modern drivers it will be 5.33-ohms.  Markbass have just rounded it up to 6-ohms so as not to confuse the uninitiated.

There is no way you could rewire it to anything other than multiples of 2.66-ohms.

You can safely use that with a valve amp on a 4-ohm setting without any problem

[Dan Dare]

It looks to me as if the valve amp the OP is considering needs to see a minimum of 4 ohms (many do). If that is the case, you can safely use it with a cab of higher impedance, but should go cautiously with lower (2.66 might be pushing things a bit, unless the output transformer is up to the job or tapped to deliver into 2 ohms). So your 6 ohm cab will be fine. You may get slightly less output, but it will be hardly noticeable and we all know that valve watts are much louder, don't we 😀?

[Bill Fitzmaurice]

1 hour ago, Dan Dare said:

It looks to me as if the valve amp the OP is considering needs to see a minimum of 4 ohms (many do). If that is the case, you can safely use it with a cab of higher impedance,

Valve amps don't have a minimum impedance load, they have a maximum impedance load, the opposite of SS amps.

[la bam]

I'd read that say if the amp head says 4 ohms then it will run ok on 6 ohms, but you'll severely hamper the life span of the valves?

[Beer of the Bass]

1 hour ago, la bam said:

I'd read that say if the amp head says 4 ohms then it will run ok on 6 ohms, but you'll severely hamper the life span of the valves?

It may be a little harder on the valves, depending on the amp and how hard you run it, but it's unlikely to be very severe with a smaller mismatch like that. 

[Dan Dare]

20 hours ago, Bill Fitzmaurice said:

Valve amps don't have a minimum impedance load, they have a maximum impedance load, the opposite of SS amps.

I didn't know that, Bill. I assumed, as you are warned on pain of destroying valve amps not to use them with no load connected, that very low impedance would be bad for them. Thanks for the heads up.

Edited December 31, 2018 by Dan Dare

[Bill Fitzmaurice]

1 hour ago, Dan Dare said:

you are warned on pain of destroying valve amps not to use them with no load connected, that very low impedance would be bad for them. Thanks for the heads up.

No load connected isn't very low impedance, it's very high impedance. It's not quite infinite, as there would be some load provided by the output transformer itself by losses within the windings, but almost. All of the power produced by the tubes still goes into the output transformer primary windings, but with no speaker connected it has no way of flowing out the secondary windings. Bad things ensue. That's also why too high an impedance load can injure both the output transformer and tubes. The power put into the primary windings can't flow out of the secondary windings as easily as they should. SS is the opposite. With no load no current passes, so no harm no foul. With too low a load impedance too much current passes, stressing both the output devices and the power supply.

[grenadillabama]

Another way a 610 cabinet may be wired is 8 ohm speakers with each pair connected in series for 16 ohms. Then the 3 pairs are connected in parallel for something like 5 ohms. I think that is what my Ashdown ABM 610 has. Any amp will drive this cabinet well.

[itu]

In general valve/tube amplifier output needs a load and transistor/solid state does not.

This can be seen so, that:

- transistor output can be open (hi-Z, no load, infinite resistance/impedance) and

- valve output can be shorted (lo-Z, short circuit, zero resistance/impedance).

Once more: this is just a generalization. Like some transistor amps need that load. Study your amp's manual.

Impedance is usually told by one number but it is a variable that depends on frequency. So if your amp and speaker state some impedances, and they are roughly in the same ballpark (4 - 8 or 8 - 16 ohms), they will work together. Very big differences or high wattage may stress especially the power amp, like Bill Fitzmaurice wrote earlier.